FIG. 1 illustrates schematically a device designed in accordance with the provisions of document FR 2 792 854 for treating containers one by one, which device is in the form of a treatment station 1 suitable for low-pressure plasma deposition of a coating on the internal surface of a container made of a thermoplastic, such as preferably, but not exclusively, a bottle made of polyethylene terephthalate (PET), which coating to be formed may consist of hydrocarbon-based or silica-based material.
Briefly, the station 1 comprises an outer cavity 2 made of conducting material, especially a metal. The cavity 2 is cylindrical with an axis A and has dimensions so as to promote a particular coupling mode of a microwave-type electromagnetic field. A generator 3, which is placed outside the cavity 2, is capable of delivering an electromagnetic field in the microwave range, the frequency of which may be typically 2.45 GHz or 915 MHz, the generator 3 being of the UHF (Ultra-High Frequency) electromagnetic wave generator type. The electromagnetic radiation delivered by the generator 3 is brought into the cavity 2 by a radially extending tunnel-shaped waveguide 4 which emerges, via a rectangular window 5 made in the cavity 2, at approximately mid-height of said cavity. The shape and the dimensions of the waveguide 4 are themselves also adapted so as to allow favorable coupling of the microwave field in the cavity 2.
Placed inside the cavity 2 is an envelope 6, which is coaxial with a cavity 2 and substantially transparent to the microwaves, which envelope defines, inside the cavity 2, a cylindrical enclosure 7 coaxial with the cavity 2. In practice, the envelope 6 is for example made of quartz. The enclosure 7 is closed off at the bottom by a bottom transverse wall of the cavity 2 and at the top by a lid 9 intended to form a sealed closure in such a way that a vacuum can be created in the enclosure 7. The container 10 to be treated is placed so as to be substantially coaxial with the cavity 2 and with the enclosure 7.
In the embodiment in question, the lid 9 is removable so as to allow the introduction of the container 10 to be treated into the enclosure 7. However, it would also be possible to provide a fixed lid and a removable bottom made in the bottom transverse wall of the cavity 2 in order to introduce or remove the container from below. Provided on the lid 9 are means 11 for supporting the container 10 by its neck 12, means for creating various levels of vacuum in the enclosure 7, and means for injecting, into the container 10, a reactive fluid that contains at least one precursor of the material that it is desired to deposit on the internal wall of the container, said injection means comprising an injection tube 13 extending partly into the container.
The device also includes respective upper and lower annular plates 14, 15 of axis A, which are placed in the cavity 2 around the envelope 6. The two plates 14, 15 are offset axially one with respect to the other so as to be placed axially on either side of the window 5 via which the waveguide 4 emerges in the cavity 2. However, their respective axial positions may vary according to the shape of the container 10 to be treated. The plates 14, 15, which are made of an electrically conducting material, are intended to form short-circuits for the electromagnetic field formed in the cavity 2 so that the field is confined axially in order to obtain a maximum intensity in the effective treatment zone.
However, the plasma coating process in a device as described above, although operating correctly, suffers from certain risks that may result in nonuniform deposition of the coating.
This is because it is often difficult to precisely control the ignition of the plasma and its stability, an unstable plasma incurring risks of forming a nonuniform coating on the internal wall of the container.
At the present time, to check whether a plasma is stable or not, the luminous intensity in the container is measured using a luminosity sensor. If a luminous instability is detected, it is therefore considered that the plasma was unstable and therefore, arbitrarily, that the coating was not deposited correctly. The container is then rejected from the production line.
Thus, if the deposition of the coating takes place by means of an installation for internally coating several thousand containers per hour, it is necessary to reduce as far as possible the percentage of containers that are not correctly coated, which represent scrap and possibly amounting to several hundred containers per day. Presently, the percentage of containers scrapped, since potentially they are not correctly coated, is around 0.5%. It would be particularly advantageous to lower this percentage so as to reduce the scrap generated by such an installation and, consequently, to reduce the corresponding loss of raw material.
Moreover, when the electromagnetic waves are transmitted into the cavity, they are reflected by the injection tube and tend to rise up along the latter toward the center of the treatment device, incurring risks of malfunction of the device and loss of electromagnetic energy that can be used in the treatment cavity, hence risks of plasma instability.
The present invention therefore relates to a device for coating an internal wall of a container, allowing both optimum distribution of the precursor gases in the container, in order to form a uniform coating, and prevention of the electromagnetic wave from rising up along the injection tube.
Thus, the present invention relates to a device for depositing a coating on an internal wall of a container making it possible to initiate and facilitate the formation of the plasma during transmission of the microwaves, and also to prevent the loss of electromagnetic energy that can be used in the treatment cavity, resulting in the formation of a more stable plasma and in the creation of a production line for internally coating containers with a percentage of containers scrapped that is substantially lower than 0.5% and advantageously tending toward a zero percentage of scrapped containers.